This invention relates to improvements in an electrophotographic process called commonly a "TESI (transfer-of-electrostatic image) process" and more particularly to improvements in a photosensitive plate used with such an electrophotographic process.
Electrophotographic processes convert a particular optical image to a corresponding electrostatic latent image, transfer the electrostatic latent image to a dielectric recording medium and to develop the transferred electrostatic latent image on the record medium by using, for example, the toner powder. One of such electrophotographic processes, which is called a TESI process is disclosed and claimed in Japanese Pat. No. 238,843. According to the cited patent, a first main surface of a photoconductive layer of a predetermined conductivity is disposed opposite to a dielectric recording medium with a very narrow gap formed therebetween while the second main surface of the photoconductive layer is irradiated with light representative of an object to be recorded (which light may be called hereinaftr the "imaging light") resulting in the completion of the recording process. More specifically, this irradiation of the photoconductive layer with the imaging light causes the generation of hole-electron pairs in the material of the layer. Among the hole-electron pairs thus generated in the photoconductive layer those carriers identical in polarity to the majority carriers present in the photoconductive layer are transferred to the first main surface of the photoconductive layer until they form an electrostatic latent image corresponding to the optical image to be recorded. Then the electrostatic latent image thus formed is transferred to the recording medium formed of a dielectric material through the very narrow gap. This results in the formation of an electrostatic latent image corresponding to the optical image on the recording medium.
In order that among the hole-electron pairs generated in the photoconductive layer those carriers identical in polarity to the majority carriers present within the photoconductive layer are transferred to the first main surface of the layer and also in order to transfer the electrostatic latent image formed on the first main face of the photoconductive layer to the recording medium, it is required to apply across the photoconductive layer and recording medium a recording voltage with a predetermined polarity from a source of recording voltage. To this end, this recording source has a first terminal connected to the record medium through a suitable voltage path and a second terminal connected to a transparent electrode layer disposed upon the second main surface of the photoconductive layer. As the second main surface of the photoconductive layer must be irradiated with an imaging light that electrode layer should be formed of any suitable transparent, electrically conductive material.
In order that, among the hole-electron pairs generated in the photoconductive layer, those carriers identical in polarity to the majority carriers present in that layer are moved to the first main surface of the layer, a certain relationship should be provided between the conductivity of the photoconductive layer and the polarity of the recording source. That is, for P type conductivity photoconductive layers, the recording source should have a polarity such that the second terminal thereof connected to the transparent electrode layer on the second main surface of the photoconductive layer has a positive polarity while the first terminal thereof coupled to the recording medium has a negative polarity. With the recording source having a polarity as above described, holes of the hole-electron pairs generated in the P type photoconductive layer are transferred to the first main surface of the layer. On the other hand, for N type conductivity photoconductive layers, the respective terminals of the recording source should be reversed in polarity from those above described in conjunction with the P type photoconductive layers. In the latter event, electrons of the hole-electron pairs generated in the N type photoconductive layer are transferred to the first main surface of the layer.
If the relationship reversed from that above described is provided, it is difficult to form the required electrostatic latent image. For example, where the photoconductive layer has a P type conductivity, the application of a recording voltage to the transparent electrode layer from the recording source to put the electrode layer at a negative potential will cause electrons of the hole-electron pairs generated in the photoconductive layer to be transferred to the first main surface thereof. During their transfer, the electrons are apt to be recombined with the majority carriers or the holes present within the P type photoconductive layer resulting in their extinction. As a result, it becomes difficult to produce the required electrostatic latent image on the first main surface of the photoconductive layer.
It has been found that the required relationship as above described gives one undesirable result. For example, for the P type conductivity photoconductive layers the transparent electrode layer should be put at a positive potential. This results in the occurrence of the phenomenon that the majority carriers or the holes from the transparent electrode layer are injected into the photoconductive layer regardless of the particular imaging light irradiating the latter layer. This phenomenon can also occur on a dark portion of an optical image formed on the transparent electrode layer. As a result, background noise is produced to decrease the signal-to-noise ratio of the resulting electrostatic latent image or visible image developed on the recording medium leading to a reduction in contrast of the final recorded image. That undesirable phenomenon also occurs in the case of N type photoconductive layers. This is because the transparent electrode layer is put at a negative potential and therefore injects majority carriers or electrons into the photoconductive layer therefrom.
Accordingly, it is an object of the present invention to provide a new and improved TESI electrophotographic process of forming an electrostatic latent image having an improved signal-to-noise ration on the recording medium involved by decreasing the background noise of the electrostatic latent image.